I got expansion boards from ebay from a link just like this one: DRV8825/A4988 42CH Stepper Motor Driver Expansion Board For 3D Prin ju | eBay . Those expansion boards do not have the extra 2 pins that the TMC2209/2226 drivers have so I think you will have to use 2208 or earlier drivers for this or find a better expansion board. Then it is just a matter of looking at a pinout of your SKR board and connecting the step and dir pins and also bringing 5v and gnd over to the board. I think I grabbed power and or gnd from one of the exp headers and then step and dir pins were right next to the stepper. And here is that pinout..